Surface effect ships (SES), as is well known, comprise a superstructure which is supported on a cushion of air that is contained by rigid sidewalls and flexible seals fore and aft sometimes called a plenum chamber. As long as the fan is on and the Cushion of air is maintained, the SES will remain in its proper attitude with respect to the water's surface. When the fan is turned off, however, there is insufficient buoyancy in the sidewalls and fore and aft seals to maintain the SES in its former attitude; the craft will thus sink to a fairly low point in the water.
Obviously, the fan could be left running at all times to maintain the craft in its proper position on the water's surface. However, this would consume large amounts of fuel if the craft were to be docked for any great length of time. Therefore standard practice is to have all docking and loading facilities for an SES or air cushion vehicle on land, so that the craft is in the water only when its fan is on and providing the cushion of supporting air. The docking means for a craft whose cushion of air is contained within a flexible skirt is generally a flat surface which can withstand the weight of the craft. The craft is driven onto this flat surface, and the fan is turned off. As air cushion pressure decreases, the craft gradually settles down onto the flat surface until its weight is supported by the rigid structure of the craft.
This type of facility cannot be used with an SES for the following reasons. First, the sidewalls of an SES are not designed to withstand the weight of the entire vehicle; it would be possible to design the sidewalls for this task, but this would then decrease the cargo carrying capacity of the SES. Second, the sidewalls of an SES extend below the level of the fore and aft seals; when the craft is in the water, the lower parts of the sidewalls are below the water's surface and the fore and aft seals are right at the water's surface. If an attempt is made to drive an SES onto a flat ramp that extends out of the water, a large gap will form between the fore seal and the surface of the ramp. Ultimately, this gap will become large enough to allow all of the air in the air cushion to escape; at this point the SES will be grounded at a point that is midway between being fully in the water and fully out of the water.